1. Technical Field
The present disclosure relates to a device to synchronize the change of the driving modality of an electromagnetic load, especially a voice coil motor used in applications for computer hard disks.
2. Description of the Related Art
In the state of the art, it is known that the reading and writing heads of a hard disk for computers are moved by a voice coil motor.
The linear modality current control of a voice coil motor in hard disk applications is accomplished by means of a power stage in a bridge configuration operating in class AB for the known properties of good linearity and cross distortion.
In order to limit the power dissipated by the output stage, during the track search operations, systems of PWM current control are used.
A method to PWM drive a power stage in a bridge configuration is described in Patent EP 760552. FIG. 1 shows the diagram of a power stage 1 with driving circuits 2 and voice coil motor 3 as described in the aforementioned patent. Power stage 1 comprises two half-bridges of transistors M1-M2 and M3-M4 arranged between voltage VM and mass GND and controlled by driving circuits 2; the voice coil motor is coupled between the shared terminal of the transistor bridges and is therefore driven through the outputs VCM_+ and VCM_−. Each driving circuit 2 is adapted to drive the corresponding output VCM_+ and VCM_− by means of a pulse width-modulated signal obtained by means of comparators adapted to compare the two 180° phase-shifted triangular voltages Tri and Tri180, produced by means of appropriate oscillators, for example, with a signal Eout. The signal Eout is produced by an error amplifier 4 having a signal Vref on the non-inverting input terminal and a signal existing on the inverting input terminal and corresponding to the difference between the current Ivcm flowing on the voice coil motor 3, detected by a resistance Rs and an appropriate amplifier Gs, and an external signal Dout. The signal Dout corresponds to the desired current value on the outputs VCM_+ and VCM_−. Specifically, the signal Eout corresponds to the difference between the detected current and the signal Dout.
The peculiarity of this system is that the current in the load is controlled by varying the duty cycle of the two outputs VCM_+ and VCM_− of stage power 1; the null current condition is obtained by driving the two outputs with two signals having the same frequency and 50% duty cycle. By increasing the duty cycle of the output VCM_+ and decreasing the duty cycle of the output VCM_−, or vice versa, the result is that the current will pass through the load with direction and intensity depending on the difference in duty cycle between the two outputs.
FIG. 2 shows the timing diagrams of the signals Tri, Tri180, Eout, VCM_+, VCM_−, Ivcm and the pulse signals Tri-Peak and Tri-Mid which show the (positive and negative) voltage peak Tri and the crossing point between the voltages Tri180 and Tri, respectively; the signals describe the operation of the current control in case of positive current in accordance with the apparatus in FIG. 1.
This method results in a reduction of the power dissipated by the power stage by means of the change of operation modality of the power stage from linear to PWM which occurs by means of an external signal L/P sent to the driving circuits 2.
If the voice coil motor 3 is controlled so that the reading and writing head moved by the same follows a track and allows to read and/or write data on the disk (“tracking mode”), the current required for this operation is of a low value, and power stage 1 is therefore controlled in linear modality.
If the reading and writing head moved by the voice coil motor 3 should operate a track skip to read new data (“seeking mode”), the current required to rapidly accelerate and brake is of a far higher level than in the case of the tracking mode. Therefore, in order to increase the efficiency of the system and decrease the power dissipated by the output stage during the track skip or seeking, the voice coil motor 3 is controlled in PWM modality to then return to linear modality at the end of the decelerating step, where the heads reach the track to be read and the current to be controlled is of a low value.
In FIG. 3 there is depicted a typical profile of the current Ivcm in the voice coil motor 3 during a seeking operation in accordance with the known art: the current to be controlled is of a high value in order to rapidly accelerate and decelerate, and the current control operates in PWM modality in order to decrease the power dissipated by the output stage.
Once the heads are close to the track on which the data are read or written, both the speed of the motor and the current controlled therein are of a low value, whereby the operation modality is changed into linear modality LIN by means of the signal L/P.
It is therefore apparent that there is a transition from one operation modality to another at the end of the seeking step.
FIG. 4 shows the voltages VCM_+, VCM_−, the current Ivcm and the signal Eout during the transition from the current control in pulse width modality (PWM) to the control in linear modality LIN in the control apparatus in FIG. 1. Upon the transition, the current Ivcm in the voice coil motor 3 has a hole or glitch due to the adjustment time needed by the control apparatus during the transition from the modality PWM to the linear modality LIN.
FIG. 5 shows the signals in FIG. 4 in more detail.
In both of the operation modalities, the mean value of the controlled current is the same. When operating in modality PWM, the current Ivcm is characterized by a ripple which is a function of the frequency of signal PWM and by the features of inductance and resistance of the voice coil motor 3. The modality change from PWM to LIN takes place asynchronously as compared to the PWM frequency at which the power stage 1 is operating. Therefore it may happen at any time during the period of the ripple of the current Icvm in the voice coil motor 3. In the specific case of FIG. 5, the change of the operation modality takes place exactly at the end of a recirculating step, where the current is at its minimum value. Under these conditions, the current glitch due to the adjustment time required for the modality change from PWM to LIN is emphasized. This current glitch is a discontinuity existing in the power-assisted control system which places the heads on the disk tracks and which is moved by the voice coil motor 3, and may deteriorate the system performance in terms of accuracy and arrival time on the tracks.